1. Field of the Invention
The present invention is directed in general to nuclear magnetic resonance tomography (MRT) as employed in medicine for examining patients. The present invention is more specifically directed to a magnetic resonance tomography apparatus with a device for graphic planning of angiographic MRT measurements that are made using a contrast agent.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been utilized for more than 15 years as an imaging method in medicine and biophysics. In this examination method, the subject is exposed to a strong, constant magnetic field. As a result, the nuclear spins of the atoms in the subject align, these having been previously irregularly oriented. Radiofrequency energy can then excite these “ordered” spins to a specific resonance. This resonance generates the actual measured signal in MRT that is picked up by suitable reception coils. The signals from examination subject can be spatially encoded in all three spatial directions by employing non-uniform magnetic fields generated by gradient coils, generally referred to as “location encoding”.
The acquisition of the data in MRT ensues in k-space (frequency domain). The MRT data in the image domain are operated on means of a Fourier transformation to produce the k-space data. The location encoding of the subject that k-space erects ensues by means of gradients in all three spatial directions. A distinction is made between the slice selection gradient (determines an exposure slice in the subject, usually along the z-axis), the frequency encoding gradient (determines a direction in the slice, usually the x-axis) and the phase encoding gradient (determines the second dimension within the slice, usually the y-axis). The selected slice can be subdivided into further slices by phase encoding along the z-axis.
Thus, a slice first is selectively excited, for example in the z-direction, and a phase encoding can be implemented in z-direction. The encoding of the location information in the slice ensues by a combined phase encoding and frequency encoding by means of these two, aforementioned orthogonal gradient fields, which, for the example of a slice excited in the z-direction, are generated in the x-direction and the y-direction by the aforementioned gradient coils.
In order to measure an entire slice of the examination subject, the imaging sequence (for example, gradient echo sequence such as the FLASH sequence) is repeated N times for different values of the phase encoding gradient, for example Gy. The time spacing of the respective RF excitation pulses is referred to as the repetition time TR. At every sequence execution, the magnetic resonance signal (for example, the gradient echo signal) is likewise sampled, digitized and stored by a Δt-clocked ADC (analog-to-digital converter) N times in equidistant time steps Δt in the presence of the readout gradient Gx. In this way, a number matrix that is produced row by row (k-space matrix or k-matrix) is obtained with N×N data points. An MR image of the slice under observation can be directly reconstructed with a resolution of N×N pixels from this dataset by means of Fourier transformation (a symmetrical matrix with N×N points is only an example; asymmetrical matrices also can be generated). For physical reasons, the data entries (values) in the region of the center of the k-matrix mainly contain information about the contrast and the entries in the edge region of the k-matrix mainly contain information with respect to the resolution of the transformed MRT image.
Tomograms of the human body can be acquired in all directions in this manner. MRT as a tomographic method in medical diagnostics is distinguished as a “non-invasive” examination method. Particularly in the case of angiographic exposures (i.e. exposures of the blood vessels in the human body, specifically in organs with blood circulation), limits exist as to the contrast that can be obtained by non-augmented MR imaging. These limits, however, can be considerably expanded by utilizing contrast agents. The functioning of contrast agents in magnetic resonance tomography is generally based on an influencing of the parameters that determine the contrast such as, for example, the longitudinal relaxation time T1 or the transverse relation time T2. Tri-valent gadolinium Gd3+, which has a T1-shortening effect, has prevailed in clinical applications. Gadolinium losses its toxicity by being bonded in chelate complexes (DTPA, diethylene triamine pentaacetic acid), so that Gd-DTPA usually can be intravenously applied. A vein is selected that that leads directly to the heart and that ultimately distributes the contrast agent in the entire arterial system. In standard sequences (T1-weighted spin echo sequence, gradient echo sequence, etc.), the accelerated T1 relaxation causes a boost of the MR signal, i.e. a brighter presentation of the appertaining tissue in the MR image. Sharp and high-contrast images of vessels in, for example, the head, neck, heart or kidneys can be obtained in this way.
Such a contrast agent-supported method in magnetic resonance tomography is generally referred to as “contrast enhanced MR angiography” (also contrast enhanced MR angiography, CE MRA). The quality of contrast agent-supported vascular exposures is significantly dependent on the time coordination of the sequence steps characterizing the measurement (data acquisition), which is generally referred to as timing or contrast agent timing. The most critical sequence steps are: contrast agent injection, measurement duration as well as measurement of the middle of the k-space matrix. The goal for achieving an optimally good contrast is to cause a maximum contrast agent concentration to be present in the region of interest to be acquired during the measurement of the middle region of the k-matrix. For this reason, contrast enhanced angiography is implemented conventionally in the following way:
1. First, overview exposures (“localizers”) of a wide greatest variety of slices are acquired in order to roughly determine the position of the vascular system of interest and to derive optimum exposure slices therefrom.
2. Injection of a test bolus is implemented wherein the time curve of the contrast agent enrichment in the region of interest (ROI) is determined. To that end, a very small dose (approximately 2 ml) of contrast agent is intravenously injected at time T2 (FIG. 2), and the MR intensity of an artery situated in the ROI is subsequently measured (per second as a rule). Using evaluation software, the intensity behavior of the contrast agent 30 in the ROI can be presented, as shown in FIG. 2. The time from the beginning of the contrast agent injection T2 to the time T3 at which the contrast agent has concentrated in an adequate amount (A, B—usually 75-80% of the maximum value) is generally referred to as the transit time or bolus arrival time, BAT. On the basis of the BAT, the user subsequently calculates the delay T4 after which the actual measurement protocol (for example, the spin echo or gradient echo sequence) should be started—referenced to the point in time of the injection T2 or Tinj. For calculating the delay time (FIG. 2), the user conventionally uses a region-dependent empirical value or a trusted equation. Possible equations employed are:Delay=BAT−TA/4Delay=BAT+Tinj/2−TA/2Delay=BAT−TTC+15%TA etc.wherein TA designates the overall measurement time (T4 through T60 of the sequence employed and TTC (time to center) references the time following the sequence start T5 at which the center row of the k-matrix is measured. As shown in FIG. 2, the delay (T2 through T4) should be ideally selected or calculated such that TA occurs in the maximum region of the concentration (time between A and B) and additionally so the center row of the k-matrix is measured after the time TTC (T5) with maximum contrast agent concentration in the ROI. The test bolus measurement thus serves for preparing for the actual measurement, i.e. the time planning in order to be able to optimize the contrast of the actual CE MRA measurement.
3. After the test bolus measurement, a pre-contrast measurement is implemented, i.e. an MR measurement without a contrast agent. In such a basic exposure without a contrast agent injection, signals are acquired from tissue in the ROI that is of no interest, but which will likewise be seen in the following contrast agent exposure (post-contrast measurement). This tissue is calculated out in the last step of the CE MRA method by means of a subsequent subtraction of the pre-contrast and post-contrast measurements.
4. The administration of the contrast agent in an increased dose (approximately 20 ml) then ensues manually at the end of the pre-contrast measurement.
5. The post-contrast measurement, i.e. the start and the execution of the selected or set MR sequence, ensues after the calculated or set delay time.
6. In the last step of the CE MRA, the exposures (scan results) of the pre-contrast and post-contrast measurements are subtracted at the image level in the form of a post-processing.
A CE MRA measurement as described above and conventionally implemented is characterized by an extremely deterministic executive sequence. The time planning of the measurement procedure is based essentially on rigid formulas without taking further physiological factors into consideration. This can result in the measurement not ensuing at the optimum point in time. If the measurement is started too early, then the center region of the k-matrix contains the contrast information measured at a point in time at which the contrast agent concentration in the ROI is not yet optimum. The consequence thereof is a poor image quality due to the appearance of artifacts in the form of edge oscillations (Gibbs ringing) that make the measurement unusable. If the measurement is started too late, the enrichment of contrast agent that is already occurring in the venous part of the vascular system leads to a superimposition of veins and arteries in the exposure and the exposure thus is likewise unusable.